In a conventional on-vehicle motor driven antenna apparatus for receiving an AM band signal and an FM band signal, a telescopic-shaped multi-stage antenna element is used, and a tip portion of a rack cord is coupled to a base end of the topmost element. Then, a pinion gear coupled to a motor drive unit is geared with the rack of this rack cord, and the rack cord is moved by either the normal rotation or the reverse rotation of the motor drive unit. As a result, the multi-stage antenna element is projected, or moved to be stored. This multi-stage antenna element is so arranged that a project length of this antenna element is set to be approximately 1 m in order that the FM band signal can be resonated, and also the rack cord owns a length of approximately 1 m. Therefore, under storage condition of the multi-stage antenna element, the rack cord is stored in a case such that this rack cord is wound on a winding drum which is rotatably provided by the rotary shaft.
There are high risks that since the projection length of this multi-stage antenna element is long, this multi-stage antenna element is broken by receiving unexpected strong force, and furthermore, this broken multi-stage antenna element may scratch other appliances. Also, it is not desirable in view of design aspect that the rod-shaped member having the length of approximately 1 m is projected from the vehicle body. Also, apparently, such a long rack cord owns flexibility so as to be wound/stored. On the other hand, this long rack cord must own the anti-buckling characteristic in order that the multi-stage antenna element is pushed up so as to be brought into the projected condition. As a result, since there are two different natures of the flexibility and the anti-buckling characteristic, the material of the rack cord and the size thereof are considerably restricted. Moreover, even when there is such a rack cord designed to realize a better balance between the flexibility and the anti-buckling characteristic, in the case that excessively large loads are given to this rack cord because of the problem of this multi-stage antenna element, or the characteristic is deteriorated due to lifetime, unbalance conditions occur as to the flexibility and the anti-buckling characteristic. As a result, there is such a problem that malfunction readily occurs.
As a consequence, the present invention provides the following motor driven antenna apparatus. That is, although the antenna effective length of the antenna element is equal to about 1 m similar to that of the prior art, the physical length thereof can be shortened, for instance, 15 cm, which is realized by the helical coil. When this very short antenna element is employed, this antenna can be hardly damaged and also can hardly scratch other electronic appliances because of such a short projection length. Also, the rack cord for projecting and storing this short antenna element can be similarly shortened, so that the rack cord can be easily designed.
As a consequence, with respect to the above-described antenna element, since the antenna effective length is equal to approximately 1 m, this antenna element can be resonated as to the FM band signal, so that a similar reception characteristic to that of the conventional long antenna element can be obtained. However, since the physical length thereof is shortened, the reception characteristic should be deteriorated as to the AM band signal. Then, an AM band signal having a low level, which is contained in the antenna output, is furthermore attenuated in the signal path defined from the antenna element to the receiver. Thus, the reception sensitivity for the AM receiver would be deteriorated. Then, there is a problem that the structure for coupling the base end portion of the antenna element to the tip portion of the rack cord would become complex, and furthermore, the mechanical strength cannot be sufficiently obtained. In addition, even when it is practically possible to obtain a sufficiently high reception characteristic under such a condition that the antenna element is completely projected, the signal level of the AM band signal would be furthermore low under such a condition that while this antenna element is projected in a half way, the physical length operable as the antenna is further shortened. When such a low level AM band signal is amplified by the receiver, noise is emphasized which may give unpleasant feelings to users. As a consequence, even when such an antenna structure is employed in which when the antenna element reaches the complete projection condition, the base end of the antenna element is elastically made in contact with the power supply member, no signal is applied to the amplification means until the antenna element is brought into the complete projection condition. Therefore, there is such a risk that the noise is produced from this amplification means.